Electric valve protective system



Oct. 12, 1943. H. WINOGRAD 2,331,643

I ELECTRIC VALVE PROTECTIVE SYSTEM Filed July 19, 1941 Patented Oct. 12, 1943 STATES BATENT OFFICE 2,331,643 mora s Yawn TE T sYsr M liar-old Winograd, Milwaukee, Wis., assignor to Allis-Chalmers Manufacturing Company, Mil- Waukee, Wis., a corporation of Delaware Abs n h Ji l 19, 1941, saw warn l6 Claims.

This invention relates in general to improvements in electric valve protective systems and m re pa t cula ly o m a s f e i a n unbalanced operation of parallel groups of electric valves i h ma e o at d i i en ndcntl of each other. Y

c ri tran atin rstcmscomn isinse ectric valves. a e frequentl qu red. o sup ly 9 rent to electric circuits or which the cont nu t of operation is a prime consideration. For exmple rectifyi s s em ma upp ir c rent to liftin elect omaenets n which an a cidental interruption of the current results in the magnet dropping its load. However, electric valves are subject to, occasional disturbances resulting from failure of valve action, and it is therefore advan a eous to arrange the valves in a plurality of parallel groups of rectifying circuits and selectively to interrupt the operation of any group in which a disturbance o cur Withut interf ing with the o erat o of he r ma nin roups. The entire y em ho ld be rendared in perative o y n the v nt f t e fa lu of the, means for interrupting the operation of a f tive r up o va ves- In qr s to r u e the number of valves to a minimum the valves o each sm e re rereisra l no dupli a ed a h lqs al a ves rnat durin s iis 'en pon sa h row may h w ver e div d into sub groups n t e P 9? ma n e ed r c nsl l to nb anc in th w rms i ea h ub-smut by s it h eh an cmcnt of t e onnect o s r ci- It is therefore one or the objects of the present invention to provide a protective system for a":

lu lit i Paral e a ms at cl tt i va v s w reic d rin d f nt han wit ys m is responsive only to occurrencc cf unbalance hetvs n h valve curr n anothe lcct (i h m ss! i nti n i to qv d a PFQ QQ V s t m o a plura o Parall g o s f lect ic al s in w c the operation of a defective group of valves'rnay be interrupted without aficcting the operation of the remaining groups.

Another object of the present invention is to provide a protective system fora plurality of parallel groups of electric valves in which the operation of the entir'system is interrupted only upon failure to interrupt the operation of a defective group of valves.

Objects and advantages other than those above described will be apparent from the followin description when read in connection with the ccompany n d aw g wh h lu t ates one cme dvicesof the system bq ms t o he pr sent in n ap e o th etr o W1 r up f e ri 'v s' onnc with an alternating current circuit through a ommon ra s rm ri Referring more particularly to the drawing by characters of reference, a polyphase alternating current circuit 6 is to be connected with a direct current circuit I through a converting system omp n a ra sf me i nd a u ali of groups of; electric valves 9, IQ, II and I2. Either ircui 6 or ci it 1 ma be c e as e s pp circuit for the system but it will be assumed that circuit 6 is a supply circuit energized from a suitable generator (not shown) and that circuit l is an output circuit transmitting current to suitable load devices (not shown). The load d vices ma he lift ng le o cns s f which the continuity of operation must'be maintained.

r n forme l m s s. a p ima w ndin is. subdivid i o a p urali o as or o connectable with the phase conductors of circuit 5 through a switch l4 serving to control the connections, between circuits ii and l by way of valves 9 to I2. Winding I3 is inductively related with a plurality of secondary windings l5, l1, l8 and H. each divided into a plurality of phase portions. Each of the phase portions of windings IE to I9 is connected with one of valves 9 to If! to form a plurality of parallel groups of rectifying circuits. The different phase portions of windings l5 and I! are relatively displaced in phase and define two neutral points connected with one conductor of circuit 1 through an interpha'se transformer 28. The different valves 9, l0 may be provided with separate cathodes or the cathodes may be combined into a common structure 2| connected with the other conductor ofcircuit 1 through a switch 22. f The dil ferent phase portions of windings [8, was s'ral hasaqi r e l k "t of windingslli ll. The terminal voltages of wind- !ll lij are fhowever, of the same magnitudes and may have the same phase displacement as the terminal voltages of windings Ii ll. The neutral points of windings l8, l9 are connected with circuit I through a second interphase transformer 2 3 The different valves ll, 12 may be provided with a common cathode 24 connected with circuit 1 through a switch 25. Switches 22 and 2,6 are provided with polarized tripping devices for causing opening of the switches only upon flow of reverse current therethrough. Suitable known means (not shown) are provided for the cathodes into electron emissive condition and for maintaining the cathodes in such condition as is Well known.

Valves 9 to E2 are provided with a dual control and protective system of which only the elements associated with valves l2 will be described in detail, only a few of the elements associated with valves 9, It! being illustrated in the drawing. The elements associated with valves H, H com-- prises a plurality of groups of control electrodes 21, 28 for controlling the conductivity of the valves. Each control electrode is connected through a resistor, such as resistor 29, with one of the secondary terminals of a control transformer 30 provided for rendering the valves periodically conductive.

Transformer 38 comprises a primary winding 3|, a secondary winding 32 and a tertiary winding 33 and may be of any type supplying voltages of sinusoidal or of peaked wave form. Winding 3| is energized from circuit 6 through connections comprising a plurality of current limiting resistors 34. These connections also preferably include a plurality of reactors 36 associated with suitable capacitors 31 for maintaining sinusoidal the wave form of the voltage impressed on winch ing 3|. Winding 32 is connected in star to provide a neutral point connected with cathode 24 through a portion of a voltage divider 38. The voltage divider receives current from circuit 6 through an insulating transformer 38, a group of rectifying devices 40 and a current smoothing reactor 4|. A capacitor 42 connected across the voltage divider cooperates with reactor 4| in reducing the flow of alternating current components through the voltage divider.

The means for rendering valves |2 nonconductive comprise suitable current responsive means such as a pair of auxiliary electric valves 43, 44. The auxiliary valves are preferably of the discontinuously controllable type in which conductive vapor is obtained from a drop of mercury. The cathodes 46, 41 of the auxiliary valves are directly connected with the negative terminal of voltage divider 38. The anode 48 of valve 43 is connected with the neutral point of winding 32 for introducing an additional negative voltage in the control circuits associated with all control electrodes 21, 28 when valve 43 is rendered conductive. The anode 49 of valve 44 is connected with cathode 41 through a group of rectifying devices 50 for operatively short circuiting winding 33 when valve 44 is rendered conductive.

Valves 43, 44 are rendered conductive only in response to the occurrence of unbalance between the currents of valves H or of valves I2. For this purpose there are provided two groups of current transformers 52 severally associated with the two groups of valves II and I2. The primary windings of transformers 5| are severally included in the rectifying circuits comprising valves II and the primary windings of transformers 52 are severally inserted in the rectifying circuits comprising valves l2. The secondary windings of transformers 5| are connected in parallel and the secondary windings of transformers 52 are likewise connected in parallel. The two groups of secondary windings are oppositely connected in parallel and are provided with common connections with an adjusting resistor 53 and with the primary Winding of a stepup transformer 54 having its secondary winding connected with a group of rectifying devices 56.

Valves 43, 44 are provided with control electrodes 51, 58 connected with the assoicated cathauxiliary electric valve 11.

odes 46, 41 through current limiting resistors 59, 68, rectifying devices 56 and a source of negative potential such as a battery 6|. A leakage resistor 62 is connected across the devices 56 to maintain a continuous circuit between the battery 6| and control electrodes 51, 58. Capacitors 63, 64 are connected between control electrodes 51, 58 and cathodes 46, 41 to prevent operation of va1ves43, 44 in response to transient voltages which may appear in the circuits of the system.

Valves 43, 44 may be returned to the inoperative condition by a resetting relay 65. Relay 66 comprises a coil 61 connected across one of the resistors 34 through a resistor 68, and is provided with a plurality of contacts 69, 1|) for short circuiting winding 3|. Relay 66 is also provided with contacts 1| controlling the connection of the coil of a time delay relay 12 across one of resistors 34. Relay 12 is provided with contacts connected across coils 61 for resetting relay 66. Contacts 13 of relay 66 are provided for short circuiting valve 43, and contacts 14 are provided for removing a short circuit across a portion of voltage divider 38.

For the purpose of causing opening of switch M in response to failure of the means above described to interrupt the flow of current through a defective group of valves, relay 66 is provided with further contacts 16 cooperating with a third Valve 11 is similar to valves 43, 44 and is provided with a cathode 18 connected with the negative terminal of voltage divider 38 and an anode 19 connected with the positive terminal of the voltage divider through contacts 16 and through the coil of a tripping relay 80. The control electrode 8| of valve 11 is connected with rectifying devices 56 through a resistor 82 and is connected with cathode 18 through a capacitor 83. Relay is provided with contacts controlling the connection of the trip coil of switch M with a suitable source of current such as a battery 84. The contacts of relay 8!) are connected in parallel with the contacts of another relay 85 similar to relay 80 and associated with the control means of valves 9, l0.

The system being connected as hereinabove described and circuit 6 being energized, transformer 39 energizes voltage divider 38 which impresses on control electrodes 21, 28 a negative potential component with respect to the potential of cathode 24 to maintain valves l2 non-conductive. Winding 32 of transformer 30 also impresses on the different control electrodes 21, 28 alternating potential components overcoming the component impressed thereon from voltage divider 38 to render valves |2 intermittently conductive.

To place the system in operating condition. switches I4, 22 and 25 are closed. Winding I8 then impresses on the diiferent valves voltages which bring the anodes of the valves sequentially to positive potentials with respect to the potential of cathode 24 to transmit current sequentially from winding l8 to circuit 1. Each valve, however, becomes effective only when the control electrode thereof is imparted a potential which is more positive than the potential of cathode 24 under the joint action of winding 32 and of voltage divider 38. Valves |2 likewise transmit current sequentially under the control of control electrodes 28. Although the phase relation between the voltages of windings I8, l8 causes the operating periods of valves H and H to be different,, the. two. groups. of; valves operate; parallel ratherthan altcmatelvas a result or themescncc of intcrphcse; hammer 2.3:.

During. n rmal oper tion, with constant. cur.- rent.the-current.imnulaostrausmittcd by the diff fcrent valves tt equal in magnitude. and in duration. "rm secondary currents of transformers t therefore. constitute a, balanced three} phase system. of currents of which the sum is equal to substantially zero at every instant The current transformer secondary windings. being ccnnectcd in parallel, each secondary winding serves; as a. return path for the other two and the windings. do not: supply any material amount of current to resistor 53. Likewise the secondary currents of transformers 52 form a balanced three-phase system of currents, and the secondary windings do. not transmit any material residual current to resistor 53.. The terminal voltage of resistor, w-hmhisimprcsaed on the primary winding of transformer 5d, therefore remains equal-to zero and valves 43, M and I! are accordingly maintained nonaconductive by battery M connected with their control electrodes.

When the magnitude of the flow of current through circuit. I varies, the flow of current through valves It changes correspondingly. During the time required forsuch change the secondary currents of transformers iii are not balanced and result in a residual current. At the same time, howeverytheflow of current through valves t2' changes by the same amount as the current of valves H, and transformers 52 also produce a residual current equal to that of transformers 5 Because thetwo groups of secondary windings of transformers 5i and 52' are connected oppositely in parallel these residual currents circulate between the secondary windings and do not appear in resistor 53.

A voltage is impressed on the primary winding of transformer 54 in response to the occurrence of unbalances between the valve currents which do not result from changes of load. If, for example, a backfire takes place in one valve H, the I current of the backfiring valve reverses while the currents of the sound valves of the same group continue to flow in their normal direction. The currents of the three valves therefore become highly unbalanced and a residual current is supplied by transformers 51 to resistor 51' The cur rents of valves 9 and Ill, however, remain balanced and therefore have no effect on the operation of the protective system.

When the residual current through resistor 53 exceeds a predetermined value. the rectified output voltage of transformer 54, whichis determined by the adjustment of resistor 53, overcomes the voltage of battery GI and brings control electrodes 51, 58 and 8| to a positive potential with respect to the potential of cathodes 46, 41, 18. The auxiliary valves thereupon become conductive. Valve 44 receives current from winding 33 through rectifying devices 50 and operatively short circuits winding 33. As a result of the inductive relation between windings SI, 32 and 33, windings 3| and 32 are likewise operatively short circuited. Substantially the full voltage of circuit 6 then appears across the terminals of resistors 34, which prevent short circuiting of circuit 6 by the opera tion of valve 44. The short circuit placed on winding 33 is not entirely without resistance and the voltage of winding 33 therefore is not quite reduced to zero. It will be understood, however, that the desired result will be obtained if the retancc oi r c y n ev ce 5;! nd. of alve 44 s ufl ciently low to. cause ho volta e o r n ormer o be reduced o a ract on of he n rmal, p r in valu s...

At th some tim v e .3 shoots a: p rt o of volta divider .3:, tb r b brinsi h pot ntials of contr e cctrodcs 2.1. as to. sub tantially t p ten ia f he ne a i e te mina rectifyin device "5- A a resul f his ac i n of he aui iarv Valves; l3. M. control e ectrodes a d 28 are continuously maintained at a. negative potential with respect: 9 the potential of cathode 24. The transto o arc between the di-ficrent sound anodes of valves it and k2 is therebyvented and the sound valves become non-conductivc. This operation takes. place within a fraction of one, cycle of the voltage of circuit 6. Durin hi period h weve a lar e rectified current. is supplied to the backfiring anode from windings l6, l1, through valves 9, H), cathode 2|, switch 22, switch 26 and cathode 24;, the circuit of the current; being completed through winding l8 and in erplw ev transformers. 2a and 2!). This current, which flows through the current coil of switch 26 in a direction reverse of the normal direction causes opening of switch 28. The switch interrupts the only current still flowing through the faulty valve, which is then rendered non-conductive. by its control electrode. Current. however, continues to be converted from circuit 6 and supplied to circuit I through winding 13, windings l6, ls'l, valves 9, H) and switch 22.

Upon interruption of the flow of current through valves H, I2, transformers 5!, 52 are all deencrgized. Control electrodes 51, 58, 8| return to a negative potential with respect to the potential of cathodes Mi, 4,1, 13,, but. valves 43, M continue to carry current because the voltages impressed thereon are unidirectional. Upon operation of valve 44 substantially the full voltage of circuit 6 is impressed on resistors 34 and coil 51, thereby causing relay Soto operate. Relay 56 closes contacts 69, Hi to short circuit winding 3L Transformer 3!! is. thereby rendered inoperative and ceases to supply current to valve 4,4, which is thereupon rendered non-conductive by control electrode 58.

Relay 66 also closes contacts 13 which short circuit valve 43 to stop the flow of current through the valve and enable control electrode 51 to render valve 43 nonrconductive. Contacts 14 remove a short circuit across a portion of voltage divider 38 to reduce the flow of current through the voltage divider. Contacts 16 connect valve I1 across voltage divider 38 but valve 11 is then non-conductive ii the flow of current through valves |I, H has been correctly interrupted and closure of contacts 15 is therefore without any effect.

Relay 66 also closes contacts H which connect coil of relay 12 across one of the resistors 34. After a predetermined time delay, relay I2 closes its contacts to short circuit coil 61. Relay is thereby caused to return to the position shown. The opening of contacts H causes relay '2 to return to the position shown. Thesystcm is then entirely returned to its normal condition and the operation of valves ll, [2 may be resumed by reclosing switch 26..-

The abnormal flow of current through valves ii, 12 may fail to be interrupted at the time of operation of relay 66 as a result either of the failure of control electrodes 21, 28 to. properly control the conductivity of the sound valves or by failure of switch 26 to open. If control electrodes Z'i', 28 fail to render all sound valves ll, l2 non-conductive, the flow of current through the valves remains unbalanced as the backfire condition then subsists. If switch 26 fails to open, valves 9, I0 continue to supply current to the defective anode and this current is rendered pulsating by the voltage of the phase portion of winding l8 associated with the backfiring anode. In either instance, therefore, current transformers 5|, 52 continue to supply a residual current to resistor 53 after continuous impression of a negative potential on control electrodes 27, 28 and after operation of relay 66.

Upon closure of contacts 16 by relay 65, valve ll is then still rendered conductive by control electrode 8! and current flows from rectifying devices 40 through the coil of relay 83, contacts 76 and valve 11, back to the rectifying devices. Relay 80 closes its contacts, thereby completing the circuit of battery 84 with the trip coil of switch It. The trip coil being energized, switch. 14 opens to disconnect transformer 8 and all valves 9, l5, ll, l2 from circuit 5. The entire system thus being put out of operation, the cause of the failure of the protective element to function properly may be investigated before returning at least the sound half of the system to normal operation. It will be understood that a disturbance in the operation of valves 9, l0 would be terminated in the manner above described by the control elements associated with these valves.

Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, current controlling means responsive to an abnormal operating condition of one of said groups for causing interruption of the flow of current through said one of said groups only, and means responsive to occurrence of failure of said current controlling means for causing interruption of the flow of current through all said roups.

2. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, current controlling means responsive only to the occurrence of unbalance between the curents of one of said groups for causing interrup tion of the flow of current through said one of said groups only, and means responsive to occurrence of failure of said'current controlling means for causing interruption of the flow of current through all said groups.

3. In an electric translating system, a'supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of rectifying circuits, a switch for controlling the connection between said supply circuit and said III [iii

load circuit by way of said valves, control means responsive only to the occurrence of unbalance between the currents of said rectifying circuits, conductivity controlling means for said valves responsive to operation of said control means for rendering said valves non-conductive, and means responsive to operation of said control means only when continued subsequently to operation of said conductivity controlling means for causing opening of said switch.

4. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of rectifying circuits, a switch for controlling the connection between said supply circuit and said load circuit by way of said valves, control means responsive only to the occurrence of unbalance between the currents of said rectifying circuits, conductivity controlling means for said valves responsive to operation of said control means for rendering said valves non-conductive, and means for causing opening of said switch comprising relay means responsive to operation of said conductivity controlling means and means responsive to simultaneous operation of said relay means and of said control means.

5. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, a switch for controlling all the connections between said supply circuit and said load circuit by way of said valves, current controlling means responsive to an abnormal operating con dition of one of said groups for causing interruption of the flow of current through said one of said groups only, and means responsive to occurrence of failure of said current controlling means for causing opening of said switch.

6. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, a switch for controlling all the connections between said supply circuit and said load circuit by way of said valves, current controlling means comprising conductivity controlling means for said valves responsive to an abnormal operating condition of one of said groups for selectively rendering non-conductive the valves of said one of said groups, and means responsive to occurrence of failure of said conductivity controlling means for causing opening of said switch. 7. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups or rectifying circuits, a switch for controlling all the connections between said supply circuit and said load circuit by way of said valves, control means responsive only to the occurrence of unbalance between the currents of any one of said groups, current controlling means comprising conductivity controlling means for said valves responsive to operation of said control means associated with any one of said groups for rendering non-conductive only the valves of said one of said groups, and means responsive to cocurrence of failure of said conductivity controlling means for causing opening of said switch.

8. In 'an electric translating system, a supply circuit, "a load circuit, means connecting 'said circuits for the transmissionof energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of rectifying circuits, a switch for controlling all the connections between said supply circuit and said load circuit by way of said valves, current controlling means comprising a second switch serially connected with one of said groups responsive to an abnormal operating condition said one of said groups for causing interruption of the flow of current through said one of said groups, and means responsive to occurrence of failure of said second switch for causing opening of the first said switch.

9. In an electric translating system, a supply circuit, aload circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, a switch for controlling all the connections between said supply circuit and said load circuit by way of said valves, means for causing interruption of the flow of current through one of said groups comprising a second switch responsive to an abnormal operating condition of said one of said groups and conductivity control-- ling means for rendering non-conductive only the valves of said one of said groups upon occurrence of said abnormal condition, and means responsive to occurrence of failure of any of said current interrupting means for causing opening of the first said switch.

10. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of rectifying circuits, and means for rendering said valves non-conductive comprising current responsive means and a plurality of current transformers having primary and secondary windings, each of said rectifying circuits comprising one of said valves serially connected with one of said primary windings, the different said rectifying circuits being connected to carry current during different periods and the different said groups being connected to carry equal currents simultaneously, said secondary windings of each said group being connected in parallel to form groups of windings oppositely connected in parallel to form groups of windings oppositely connected in parallel, and common connections between said groups of windings and said current responsive means.

11. In an'electric translating system, a supply circuit, a load circuit, means (connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves and a transformer having a plurality of polyphase windings each divided into a plurality of phase portions, and means for rendering said valves non-conductive comprising current responsive means and a plurality of current transformers having primary and secondary windings, the different said phase portions being all of different phases, each of said phase portions being connected in series with one of said valves and with one of said primary windings, and the different said polyphase windings being connected to carry equal currents simultaneously, said secondary windings associated with each said polyphase winding being connected in parallel to form groups of windings oppositely connected inparalle'l, and common-connections between said groups of windings and said current responsive means.

12. In an electric "translating system, -a supply circuit, a load circuit, means connecting said circuits for the transmision of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, a first current controlling means responsive to an operating condition of one of said groups for causing interruption of the flow of current through said one of said groups only, a second current controlling means for causing interruption of the flow of current through all said groups, and means responsive to operation of said first current controlling means for rendering said second current controlling means responsive to said operating condition.

13. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, a plurality of current controlling means severally associated with the different said groups and each responsive to an operating condition of one of said groups for causing inter ruption of the flow of current through the said one of said groups only, another current controlling means common to all said groups for causing interruption of the fiow of current through all said groups, and means responsive to operation of any of the first said current controlling means for rendering the second said current controlling means responsive to all said operating conditions.

14. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, a first circuit controlling means responsive only to the occurrence of unbalance between the currents of one of said groups for causing interruption of the flow of current through said one of said groups only, a second current controlling means for causing interruption of the flow of current through all said groups, and means responsive to operation of said first current controlling means for rendering said second current controlling means responsive to occurrence of said current unbalance.

15. In an electric translating systeln,a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, a switch for controlling all the connections between said supply circuit and said load circuit by way of said valves, current controlling means comprising conductivity controlling means for said valves responsive to an operating condition of one of said groups 'for selectively rendering non-conductive the valves of said one of said groups, control means for causing opening of said switch and means responsive to operation of said current controlling means for rendering said control means responsive to said operating condition.

16. In an electric translating system, a supply circuit, a load circuit, means connecting said circuits for the transmission of energy therebetween comprising a plurality of electric valves connected in a plurality of parallel groups of circuits, a switch for controlling all the connections between said supply circuit and said load circuit by way of said valves, current controlling means comprising conductivity controlling means for said valves responsive only to the oc currence of unbalance between the currents of one of said groups for rendering non-conductive the valves of said one of said groups only, control means for causing opening of said switch and means responsive to operation of said current controlling means for rendering said control means responsive to occurrence of said current unbalance.

HAROLD WINOGRAD.

CERTIFICATE OF CORRECTION. Patent No. 2,551,6L 5. October 12, 1915. HAROLD WINOGRAD.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows Page 5, first Column, lines 55 and 5h, claim 10, strike out "to form groups of windings oppositely connected in parallel"; and that the said Letters Patent should be read with this correction therein that the same may conform to the rec- 0rd of the case in the Patent Office.

Signed and sealed this 50th day of November, A. D. 1911.5

Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION. Patent No. 2,551,615. October 12, 19L 5 HAROLD WINOGRAD It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows Page 5, first column, lines 55 and 51;, claim 10, strike out "to form groups of windings oppositely connected in parallel"; and that the said Letters Patent should be read with this correction therein that the same may conform to the rec- 0rd of the case in the Patent Office.

Signed and sealed this 50th day of Nov-ember, A. D. 1915.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents. 

